1. Field of the Invention
This invention relates to an improvement of a sound absorbing mechanism to be placed around a noise generating source or in a propagation path of a noise, and more particularly relates to a sound absorbing mechanism using a porous material.
2. Description of the Prior Art
Prior Art 1
FIG. 44 is a sectional view showing the construction of a conventional sound absorbing mechanism using a hard porous material as a first prior art (prior art 1), and the figure also has an explanatory diagram for showing a sound pressure distribution of a sound wave to be input into the sound absorbing plate thereof. In FIG. 44, reference numeral 1 designates a sound insulator such as a wall: and numeral 2 designates a sound absorbing plate of a hard porous material made of plastic particles, a ceramic, foam metal or the like, for example. Reference numeral 11 designates a back air space of the sound absorbing plate 2; numeral 11a designates the thickness of the back air space 11; numeral 81 designates an input sound; reference character .beta. designates an average input angle of the input sound 81; and character .lambda. designates a wavelength of a sound wave having the highest sound pressure level among the input sounds 81. In the explanatory diagram showing a sound pressure distribution, mark + designates the operation of positive pressure on the sound absorbing plate 2; and mark - designates the operation of negative pressure on the sound absorbing plate 2. Arrows 85 and 86 designate directions of an input sound wave operating on the back air space 11 through the sound absorbing plate 2.
Next, the operation thereof will be described. The input sound 81 passes through the sound absorbing plate 2 to be input into the back air space 11. The sound absorbing plate 2 has acoustic mass m and acoustic resistance r as the acoustic characteristics thereof, and the back air space 11 has acoustic capacity c as the acoustic characteristic thereof. The acoustic equivalent circuit according to the acoustic characteristics of the sound absorbing plate 2 and the back air space 11 can be expressed as a series resonance circuit of r-m-c. According to this series resonance circuit, the resonance frequency thereof f.sub.0 is expressed as the following formula. EQU f.sub.0 =(1/2 .pi.).times..sqroot.(1/mc) (1)
When a sound wave having a frequency close to this resonance frequency f.sub.0 is input into the sound absorbing plate 2, the input impedance observed from the sound source side becomes minimum. Accordingly, only the acoustic resistance r of the sound absorbing plate 2 should be considered. If the acoustic resistance r of the sound absorbing plate 2 is tuned to be a value close to the characteristic impedance .rho..times.a (.rho.: density of air; a: sound velocity) of air, the sound absorption coefficient becomes 1.0 at the resonance frequency f.sub.0. Consequently, the sound wave having the frequency close to the resonance frequency f.sub.0 penetrates into the sound absorbing mechanism most efficiently. The penetrated sound wave forces the air existing in the back air space 11 and having an acoustic characteristic of acoustic capacity c to vibrate. The vibrated air goes in and out through gaps in the sound absorbing plate 2, and the sound wave is transformed into thermal energy by the acoustic resistance r of the gaps. That makes it possible to radiate energy. This means that the energy of the input sound wave was absorbed in the sound absorbing mechanism, namely sound absorption has been performed.
In the aforementioned sound absorption mechanism, it is known that the efficiency of sound absorption is highest in the case where the input sound 81 is input into the sound absorption plate 2 perpendicularly. That is to say, in the case where a sound wave is input perpendicularly, the phase relation of the sound wave on the top surface of the sound absorbing plate 2 is equal at any place on the top surface, and the whole of the sound absorbing plate 2 and the whole of the back air space 11 are unified consequently, so that the effective operation of resonance and sound absorption is performed. On the other hand, the case where the input sound 81 is input into the sound absorbing plate 2 not perpendicularly but at a certain input angle .beta. ill be considered as an ordinary case. As shown in FIG. 44, when a sound wave having a wavelength .lambda. is input into the sound absorbing plate 2 at an input angle B, a phase difference having a period of .lambda./cos (.beta.) of sound pressure distribution is generated on the sound absorbing plate 2. A sound wave is basically absorbed by utilizing a resonance phenomenon. But, if a distribution of the strength of sound pressure is generated along a direction on a surface of the sound absorbing plate 2, pressures 85 and 86 having reverse directions to each other operate on the back air space 11, so that adjoining parts of the back air space 11 is acoustically oscillated reversely. Then, pressures are balanced in the back air space 11, and consequently it becomes difficult that air vibrations synchronized with input sound waves are generated. That is to say, it becomes difficult that resonance phenomena are generated between the sound absorbing plate 2 and the back air space 11, so that sound absorption effect is extremely checked.
Prior Art 2
FIG. 45 is a longitudinal sectional view showing a sound absorbing mechanism utilizing a sound absorbing material and a resonance phenomenon by combining them as a second prior art (prior art 2), which is shown, for example, in Japanese Patent Gazette No. 76116/1992 (Tokko-Hei 4-76117). FIG. 46 is a sound absorption characteristic diagram of the sound absorbing mechanism shown in FIG. 45. In FIG. 45, reference numeral 91 designates a wall; numerals 92 and 93 designate air spaces; numeral 94 designates a small opening or a slit; numeral 95 designates a nozzle; numeral 96 designates a porous plate; and numeral 97 designates a sound absorbing material.
Next, the operation thereof will be described. The aforementioned sound absorbing mechanism of the prior art 2 is provided with a porous plate 96 apart from the wall 91 with the air space 92 between. The porous plate 96 has a large number of small openings or slits 94, which are provided with nozzles 95 connected to them. Across the porous plate 96, the sound absorbing material 97 which is made of a fibrous material or a material made of a large number of particles is set over the whole plane at the tips of the nozzles 95 with the air space 93 between. In this connection, the air space 92, the small openings or slits 94 and the nozzles 95 comprise sound absorbing mechanisms utilizing a resonance phenomenon, and the sound absorbing material 97 and the air spaces 93 comprise sound absorbing mechanisms utilizing sound absorbing materials. The aforementioned elements of the sound absorbing mechanisms utilizing a resonance phenomenon are connected to each other through the air space 92, and the elements of the sound absorbing mechanisms utilizing sound absorbing materials are connected to each other through the air space 93.
The sound absorbing mechanism of the prior art 2 has a sound absorption characteristic of the curved line 3 shown with a solid line in FIG. 46. A sound absorption characteristic of a sound absorbing mechanism utilizing only a resonance phenomenon is shown with a dotted line (curved line 2) in FIG. 46, which sound absorbing mechanism has large sound reduction effects at lower frequencies. A sound absorption characteristic of a sound absorbing mechanism utilizing only sound absorbing materials is shown with a dashed line (curved line 1) in FIG. 46, which sound absorbing mechanism has large sound reduction effects at higher frequencies.
Prior Art 3
FIG. 47 is a partially cutaway perspective view showing the construction of a conventional sound absorbing mechanism as a third prior art (prior art 3), which utilizes both the slits and a porous material and is shown, for example, at pp. 245-250 and pp. 351-356 of Kenchiku Onkyo Kogaku Hando Bukku (Architectural Acoustics Handbook) ed. by Nippon Onkyo Zairyo Kyokai (Japan Acoustical Materials Association) (Gihodo, Tokyo, 1963). FIG. 48 is a sound absorption characteristic diagram of the sound absorbing mechanism shown in FIG. 47. In FIG. 47, reference numeral 91 designates a wall; numerals 92 and 93 designate air spaces; numeral 98 designates a porous material; and numeral 99 designates a slit plate.
Next, the operation thereof will be described. The aforementioned sound absorbing mechanism of the prior art 3, which uses a structure utilizing slits and a porous material, raises the sound absorption characteristics of the porous material 98 and the air space 92 by means of the resonance phenomena of the slit plates 99 and the air spaces 93. As shown in FIG. 48, the raised sound absorption characteristics are particularly effective at lower frequencies around 200 to 500 Hz due to the resonance phenomena at the slit parts.
Since the sound absorbing mechanism of the prior art 1 is constructed as mentioned above, the resonance frequency f.sub.0 is determined in accordance with the thickness 11a of the back air space 11 if the sound absorbing plate 2 is specified. The sound absorption coefficient becomes maximum at the resonance frequency f.sub.0, and the sound absorption characteristic has large values in a narrow frequency band with the resonance frequency f.sub.0 as a 1/3 octave band center frequency. Since some sound pressure distributions are generated in some directions on the sound absorbing plate 2 when sound waves are input into the sound absorbing plate 2 at angles other than a right angle, the prior art 2 has a problem that the interference of input sound waves is generated at some frequencies according to phase differences to bring about the reduction of the sound absorption coefficient.
Since the sound absorbing mechanism of the prior art 2 is constructed as mentioned above so that a sound absorbing mechanism utilizing a resonance phenomenon to be generated by elements connected to each other and a sound absorbing mechanism utilizing sound absorbing materials connected to each other are combined to absorb sound waves, the prior art 2 has problems that some sound pressure distributions are generated in some directions on the sound absorbing material 97 when sound waves are input into the sound absorbing material 97 at angles other than a right angle similarly in the prior art 1, so that the interference of input sound waves is generated at some frequencies according to phase differences to bring about the reduction of the sound absorption coefficients at lower frequencies as shown in, for example, FIG. 46.
The sound absorbing mechanism of the prior art 3, which utilizes slits and a porous material, has a problem that the sound absorption coefficients at lower frequencies around 200 Hz to 500 Hz are large due to sound resonance phenomena at the slits but the sound absorption coefficients at higher frequencies more than 500 Hz are small.